Ablatement designed for dark adaptability

ABSTRACT

A system and method for performing a reshaping of a cornea of an eye for improved vision is presented. The system comprises a first appartus for determining dark adapted pupil size of an eye and a second apparatus for reshaping a cornea of the eye in an area approximately equal to the dark adapted pupil size as determined by the first apparatus. The method of the present invention involves dilating the pupil of an eye to its dark adapted size, determining the diameter of the dilated pupil, and ablating the cornea of the eye to match the dilated pupil size. The advantage to using such a system and method when reshaping a cornea of an eye is reduced halo effect or improved night vision.

FIELD OF INVENTION

The invention relates generally to a laser system for corneal sculpting.More specifically, it relates to a system and method for determining thedark adapted pupil size of a patient and reshaping the cornea of the eyebased on the dark adapted pupil size.

BACKGROUND OF THE INVENTION

Of the various components in the human eye, the cornea is the principaloptical element for refracting incident light onto the retina in theform of a clear image. Photorefractive keratectomy (PRK) is a procedurewhich typically utilizes an excimer laser beam to vaporize "ablate"corneal tissue in a precise manner to correct for focussing deficienciesof the eye. An excimer laser is preferred for this procedure becausepulsed ultraviolet ablation is predictable, discrete, and non-damagingto adjacent tissue. PRK generally involves mechanical removal of theepithelium or outer layer of the cornea to expose the Bowman's layer onthe anterior surface of the stroma. Laser ablation usually begins at theBowman's layer. The laser beam removes corneal tissue to varying depthsas necessary for recontouring the anterior stroma. Afterward, theepithelium rapidly regrows and resurfaces the contoured area, resultingin an optically correct (or much more nearly so) cornea. In a variationof the procedure, a surface flap of the cornea is folded aside and theexposed surface of the cornea's stroma is ablated to the desired surfaceshape with the surface flap then being replaced.

The specific region of the cornea involved in the refractive imageformation will vary with the size of the pupil. Only a small centralcorneal region will refract light onto the cornea when the pupil isconstricted under bright lighting conditions. Under dim lightingconditions, when the pupil is substantially dilated, a much largercorneal region is involved forming an image on the retina. Thisvariation in pupil size can become an issue for a PRK patient if thediameter of the laser-treated corneal region ("the optical zone") issmaller than the dilated pupil diameter. When the ablated optical zoneis smaller than the patient's dark adapted pupil size, the patient'snight vision is affected. Typically, the patient's vision will be hazyor somewhat blurred, and the patient may perceive halos around brightlights. Approximately 20 percent of patients treated with a 5 mm opticalzone have complained of such problems. This is a result of the pupilbecoming larger than 5 mm as the pupil adapts for darkness. When a 6 mmoptical zone is ablated, it is estimated that only 2 percent of patientscomplain of night-vision problems.

One apparent strategy for avoiding such night-vision problems would beto treat all patients with an optical zone larger, much larger, than themaximum pupil diameter. However, there are several disadvantages to thisapproach. First, maximum pupil diameter varies from patient to patient.Second, the maximum depth of laser ablation and the total volume oftissue removed both increase with optical zone diameter. Such increasestypically lead to more regression, that is, deterioration, of therefractive change as the cornea heals. In addition, the increased tissuevolume to be removed necessitates a longer laser procedure. Variationsin corneal ablation behavior over time due to hydration changes in thede-epithelialized tissue (known to occur) maybe degrade the accuracy ofthe ablative corneal reshaping for lengthy procedures. A much moredesirable strategy, not advanced until the present invention, is totailor the optical zone diameter in each treatment to the maximum pupildiameter of that patient.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asystem and method for determining the-dark adapted pupil size of an eye.

Another object of the present invention is to provide a system andmethod for determining the dark adapted pupil size of an eye as a toolin ophthalmic laser surgery to include corneal sculpting procedures.

Yet another object of the present invention is to provide a system andmethod for determining the dark adapted pupil size of an eye that issurgically eye safe.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a system and method areprovided for performing a reshaping of a cornea of an eye for improvedvision. The system comprises a first apparatus for determining darkadapted pupil size of an eye; and a second apparatus for reshaping thecornea of the eye in an area approximately equal to the dark adaptedpupil size as determined by the first apparatus. The preferable appartusfor determining the dark adapted pupil size of an eye permits trackingthe eye as ambient lighting is dimmed down to complete or near-completedarkness with an electrical control of a zoom mechanism. The zoom refersto adjusting the diameter of the spot pattern projected onto the eye.Feed back can be utilized to sense the return from the spots, and thento adjust the zoom so that the spot pattern diameter matches the pupildiameter. Laser ablation would then proceed over that circular region(maybe, plus a little outside). Alternatively, a video system with acircular cursor could be used in dim illumination to identify theposition and diameter of the dilated pupil. The zoom mechanism has anelectrical control for size adjustment of a beam optical radiation or aplurality of such radiation beams (either at a visible or infra-redwavelength) incident on a boundary coincident with the dark adaptedpupil size of the eye. The first apparatus may also comprise deliveryoptics for focusing a plurality of optical radiation beams on acorresponding plurality of positions located on a boundary coincidentwith the pupil size of the eye to form a pattern. Zoom optics may beused for adjusting the pattern formed by the plurality of opticalradiation beams incident on said corresponding plurality of positions.An optical receiving arrangement for detecting reflected energy fromeach of the plurality of positions is also employed to determine thedark adapted pupil size. The pattern formed by the plurality of opticalradiation beams is equivalent to the dark adapted pupil size of the eye.The dark adapted pupil size is entered into a computer program forcorneal sculpting.

In performing laser reshaping of an eye for improved vision, thediameter of the optical zone ablated on the cornea is adjusted to match,or nearly match, the diameter of the dilated pupil. The ablation profileat the periphery of the optical zone may be tapered to form a smoothtransition between treated and untreated portions of the eye.

This patent application is copending with related PCT patentapplications entitled, "Laser Sculpting Method and System",International publication number WO 95/28890; "Eye Movement SensingMethod and System", International publication number WO 95/28879; and"Laser Beam Delivery and Eye Tracking System", International publicationnumber WO 95/28989 all of which were published on Nov. 2, 1995, andowned by a common assignee of subject PCT applications. The disclosuresof these three applications all of which are based on United Statespatent applications, are incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of the system of the present invention;

FIG. 1B is a plan view of an eye showing four light spots positioned onthe eye's iris/dark adapted pupil boundary; and

FIG. 2 is a block diagram of a preferred embodiment optical arrangementfor the focusing optics of the first appartus.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1A, a blockdiagram of the system of the present invention is shown. The eye 10 isshown with an enlarged pupil 11 which has been adapted to its darkadapted size. As the lights are dimmed or the pupil is otherwiseenlarged to a dark adapted pupil size such as by dilation by appropriateeye drops, the apparatus for determining dark adapted pupil size 12emits at least one invisible beam of light onto the eye to form one ormore spots for defining the size of the dark adapted pupil. The size ofthe spot is adjusted to match the size of the dark adapted pupil 12 byfocussing on the pupil/iris boundary. The measured diameter of the spotsize is then input into a program which is used to run the corneashaping apparatus 13. The zone which is ablated under the cornealsculpting program is just a little larger in diameter than the darkdiameter of the adapted pupil to allow for a smooth transition betweenthe treated and untreated portion of the eye.

FIG. 1B shows a preferred method for determining the diameter of thedark adapted pupil size 11 of an eye 10. In this method, four spots oflight 15, 16, 17 and 18 are positioned on the boundary between the pupil11 and the iris 14. As the lights are dimmed in the room, the spots oflight are used to track the pupil as it dilates. Any movement of the eyemay also be tracked using the four spots of light as has been previouslydisclosed. The wavelength and power of the light spots can be outsidethe visible spectrum so as not to negate the dark adaptable pupil sizeor to interfere or obstruct the surgeon's view of the eye undergoing thesurgical procedure or injure the subject eye.

As a preferred embodiment, the apparatus for determining the darkadapted pupil size and the cornea shaping apparatus for carrying out themethod of the invention are described in detail in the aforementionedcopending PCT patent applications. However, for the purpose of thepresent description, the optical arrangement will be described brieflywith the aid of the block diagram of FIG. 2. In FIG. 2, fiber opticbundle 123 is positioned at the working distance of microscope objective1302. The numerical aperture of microscope objective 1302 is selected tobe equal to the numerical aperture of fibers 116, 118, 120 and 122.Microscope objective 1302 magnifies and collimates the incoming light.Zoom lens 1304 provides an additional magnification factor for furthertunability and for determining the dark adapted pupil size. The zoomlens increases the spot size as the lights are turned down to examinethe patient. Collimating lens 1306 has a focal length that is equal toits distance from the image of zoom lens 1304 such that its output iscollimated. The focal length of imaging lens 1308 is the distance to theeye such that imaging lens 1308 focuses the light as four sharp spots onthe corneal surface of the eye at the boundary between the pupil and theiris. An optical receiving arrangement detects reflected energy fromeach of the light spots which is used to measure the diameter of thedark adapted pupil size. This size measurement is subsequently enteredinto a program for corneal sculpting.

Once the size of the ablation zone is determined, the depth of ablationfor each diopter of correction can be determined. For example, if thesurgical method requires a 5 mm zone, ablation will only have to go downabout 10 microns deep for one diopter of correction. For a 6 mm zone,the ablation would go down about 13 to 15 microns deep for each diopterof correction. The treatment is customized for the needs of theindividual, but a lower limit set for approximately 5.5 mm as determinedfrom a statistical data base. Alternatively, the low end limit for theablation diameter may be the measured diameter of the dark adapted pupilsize of the individual.

This method allows for customizing the ablation zone of the patient'sdark adapted pupil size to eliminate the halo problem. In turn, thisprocedure may be done for all of the patients on a customized basiswithout having to ablate a large volume for patients who only need asmaller volume ablated if they have a smaller dark adapted pupil size.

Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in the light of theabove teachings. It is therefore to be understood that, within the scopeof the appended claims, the invention may be practiced other than asspecifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for performing a reshaping of an eyefor improved vision, the method comprising the steps of:dilating thepupil of an eye to a dark adapted size for providing a dilated pupil;determining the diameter of said dilated pupil; and ablating the corneaof said eye to a diameter approximately matching said dilated pupilsize.
 2. A method for performing a reshaping of an eye for improvedvision according to claim 1, wherein the diameter of said dilated pupilis determined by focussing a light spot at said dilated pupil of saideye to match said dark adapted size.
 3. A method for performing areshaping of an eye for improved vision according to claim 1, whereinsaid dilated pupil has a diameter determined by focussing a plurality oflight spots at the dilated pupil of said eye to form a pattern to matchsaid dark adapted size.
 4. A method for performing a reshaping of an eyefor improved vision, according to claim 1, the method further comprisingthe step of tapering said cornea to form a smooth transition betweentreated and untreated portions of said eye.
 5. A method for performing areshaping of an eye for improved vision, according to claim 1, themethod further comprising the step of inputting the determined diameterof said dilated pupil into a corneal sculpting program.
 6. A method forperforming a reshaping of an eye for improved vision according to claim1, wherein the pupil dilating step comprises the step of dimming lightreceived by said eye for darkening an environment of said eye.
 7. Amethod for performing a reshaping of an eye for improved visionaccording to claim 1, wherein the pupil dilating step comprises the stepof adding a chemical eye drop onto said eye.
 8. A method for improvingvision of an eye, the method comprising the steps of:dilating the pupilof an eye to a size adapted for vision in a darkened environment forproviding a dilated pupil; determining the diameter of said dilatedpupil; and ablating corneal material of said eye to within an ablatingdiameter approximately sized to said diameter of said dilated pupil. 9.A method for improving vision according to claim 8, wherein the pupildilating step comprises the step of dimming light received by said eyefor darkening an environment of said eye.
 10. A method for improvingvision according to claim 8, wherein the pupil dilating step comprisesthe step of adding a chemical eye drop onto said eye.
 11. A method forimproving vision according to claim 8, wherein the dilated pupildiameter determining step comprises the steps of:placing a light spot onsaid dilated pupil sufficient for covering said dilated pupil of saideye; and measuring the diameter of said light spot.
 12. A method forimproving vision according to claim 8, wherein said dilated pupil has adiameter determined by placing a plurality of light spots within saiddilated pupil for matching a pattern to said dark adapted size of saidpupil.
 13. A method for improving vision according to claim 8, whereinthe cornea ablating step comprises the step tapering said cornea to forma smooth transition between treated and untreated portions of said eye.14. A method for improving vision according to claim 8, the methodfurther comprising the step of inputting the diameter of said dilatedpupil into a corneal sculpting program.